Ch 11

Question 1

What are three actions that the plasma membrane is involved in?

Answer 1

1. receiving information
2. import / export of small molecules
3. capacity for movement and expansion

Question 2

What does amphipathic mean?

Answer 2

Both hydrophilic + hydrophobic parts
ex. phospholipids have hydrophilic heads + hydrophobic tails

Question 3

Examples of membrane lipids?

Answer 3

phosphatidylcholine (chloine group in head)
phosphatidylserine (serine group in head)
cholesterol
glycolipids

Question 4

How / why do hydrophobic molecules avoid water?

Answer 4

molecules like to be in a less energetic state –> (i.e more entropic + disordered)

When individual hydrophobic molecules are “caged” by water molecules –> it is highly ordered and requires free energy

–> therefore, they aggregate + cluster together to minimize energy cost

Question 5

Properties of phospholipids

Answer 5

• lipid bilayer: energetically favorable
• self-sealing: reverts to bilayer when severed (b/c free edge exposes hydrophobic tails to water)

Question 6

how do membrane phospholipids move?

Answer 6

1. lateral diffusion
2. flexion
3. rotation
4. flip-flop (from one mono-layer to the next mono-layer)

Question 7

Purpose of cholesterol

Answer 7

stiffen cell membranes by inserting itself between phospholipids and locking / preventing lipid movment

Question 8

Process of membrane assembly in ER

Answer 8

1. portion of membrane on ER is modified (phospholipid synthesis adds to cystolic half of bilayer)
2. protein SCRAMBLASE randomly transfers phospholipids to the other monolayer)
3. results in symmetric growth of lipid bilayer

Question 9

What happens when vessicles fuse with membrane of other organelles?

Answer 9

1. membrane delivered from ER
2. protein FLIPPASE transfers specific phospholipids to cytosol monolayer
3. Golgi apparatus + other cell membranes have asymmetric distributions

Question 10

How do membrane proteins insert themselves in/on the lipid bilayer?

Answer 10

Transmembrane (integral membrane):
1. singlepass (signal receptors) *alpha
2. multipass (channels) *alpha
3. beta barrel (channels)
- often requires 16 beta sheets

Monolayer-associated (integral membrane)

Lipid-Linked (integral membrane)

Protein-Attached (peripheral memrbane)

(transmembrane + mono-layer associated proteins insert themselves directly into membrane)

Question 11

How to isolate membrane proteins?

Answer 11

use detergents: SDS + TritonX-100

1. detergent monomers have 1 hydrophobic tail + hydrophilic head
2. separates protein from lipids in homogenous solutions (still difficult to crystalize)

Question 12

Function of cell cortex

Answer 12

meshwork of fibrous proteins that reinforces plasma membrane

• often made of spectrin –> forms a lattice and helps maintain biconcave shape of RBC

Question 13

If phospholibds move laterally, how and why do membrane proteins remain stationary?

Answer 13

Proteins are tethered:
1. to cell cortex
2. to extracellular matrix
3. to proteins on surface of another cell
4. diffusion barriers (ex. tight junctions)

Why?
proteins serve different functions –> maxmimized function at certain locations
ex. Ca2+ voltge gated channels are localized near the end of a nerve cell axon at presynaptic membrane

Question 14

Why are cells slimy?

Answer 14

coated with sugars
1. glycolipid
2. glycoprotein
3. proteoglycan